Exact positioning of tables in two directions of one plane under closed-loop control, as well as the closed-loop control of the position of the table in a direction perpendicular to the plane and with regard to rotations of the table about the three linear directions is, above all, an important requirement in the manufacture of semiconductors. In that case, a wafer placed on the table must be positioned under a tool such as the lens of an exposure apparatus, the objective lens of a microscope, or an electron-beam lens for processing or inspecting the wafer. In conjunction with a stationary tool, the table must be able to execute movements in the wafer plane on the order of magnitude of the wafer itself. At least small corrections are necessary perpendicular to that and with regard to all possible tilts to permit compensation for variations in the thickness of the wafer, for example, or deviations in the parallelism of the front side and back side of the wafer.
In this field of technology, practical applications are familiar for which the table is moved back and forth in a horizontal direction in individual swaths over the width of the wafer, after each crossing of the wafer, a jump being made to another swath, so that the wafer is scanned by the tool in meander-shaped paths. In so doing, the velocity of the table is constant within one swath. In this context, large forces are necessary primarily for the direction reversal at the end of each swath. Apart from that, the drives only have to permit small position correction movements and keep the velocity of the table constant.
Tables of the kind which are moved by magnetic force and are held in suspension (referred to as Maglev stages) are especially suitable for manufacturing semiconductors because, due to the fact that mechanical supports are not used, these tables cause only very few disturbing particles, making them especially appropriate for clean rooms.
An example for such a table is described in U.S. Pat. No. 6,879,063, which describes a system in which the table moves via a planar array of magnets. At its underside, the table has coils, via which, upon being suitably traversed by current, the table may be moved in all six degrees of freedom. However, tables of this type are not optimized for the special movement pattern described above, since their drives are unnecessarily heavy (because the large forces for accelerating are not necessary over the whole working area), and as a consequence, additional masses must be moved. In addition, the supply cables necessary for the moving coils are disadvantageous for a precise positioning. The heat generated in the coils moved with the table can be a problem for many applications. The fields of the magnets may also be disadvantageous for various practical applications, as in ion-beam or electron-beam applications.
A magnetic bearing, with which a load is able to be held in the vertical direction and positioned over small distances, and at the same time may be moved in the horizontal direction is described in PCT International Published Patent Application No. WO 98/37335. A U-shaped yoke is used as a stator, having two parallel limbs disposed above and below a movable, ferromagnetic bar. Integrated in the closed end of the U-shape of the yoke is a permanent magnet whose magnetic flux is guided along the limbs, the magnetic circuit being closed across the air gaps between the yoke and the bar. The reluctance forces occurring in this instance counteract the gravitational force of the bar and may be regulated with the aid of a coil, which is able to weaken or strengthen the field of the magnet, for the vertical fine positioning of the bar. In addition, a further coil, which is wound on one of the limbs of the yoke, makes it possible to exert a horizontal force on the bar. Several such configurations of yoke and bar, disposed relative to each other in various directions, permit the positioning of the bars and an object joined to the bars in all six degrees of freedom. The masses moved by the drive—e.g., the bars—are very small. The drive is of the moving iron type, in contrast to drives with moving coils or moving magnets.
The horizontal forces attainable by such drives represent a problem for certain practical applications, the forces possibly being too small to be able to carry out machining processes with high throughput rate. To achieve high throughput, it is necessary to slow down and stop a rapidly moving wafer over a short distance, and then to accelerate it again in the opposite direction until the rapid movement is reversed.